1. Field of the Invention
The present invention relates to a speed change operation device for a vehicle such as crawler truck, construction machine with caterpillar, cultivator or snowmobile having a continuously variable transmission to drive caterpillars or wheels and more particularly, a speed change operation device of this kind which has a simple construction, can be operated easily and provides the vehicle with good running and turning characteristics.
2. Description of the Related Art
In the crawler truck, construction machine with caterpillar, cultivator or snowmobile having right and left caterpillars, it is necessary to continuously change the revolving speed of the right and left caterpillars in order to change the direction of advance. Speed change operation devices to meet such demands have been proposed by Japanese Laid-Open Patent Publications Nos. 9-30278 (see FIG. 9), 7-323864 and 7-305442.
However, in those conventional speed change operation devices, the construction is complex and a plurality of rotation support shafts for pivotally supporting a plurality of movable members such as links, plates or the like constituting the speed change operation device are disposed to three-dimensionally cross each other in a complicated manner so that there are many parts producing rotary wear. The speed change operation devices are somewhat inferior in respect to their operation and wear resistance, where the size of movable parts such as links, levers, plates and the like must be increased thereby reducing the accuracy of the speed change control and increasing the size of the device.
Referring to FIG. 9, 010 denotes a change lever for setting the speed and advancing direction of a vehicle. Rotation of the change lever 010 is transmitted to a rotary shaft 013 through a rod 012 and an arm 011, and a bracket 014 integral with the rotary shaft 013 and a mixing lever 016, which is supported by the bracket so as to rock about a pivot pin 015 in a plane parallel with the surface of the drawing, are rocked in a plane perpendicular to the surface of the drawing.
On the one hand, a guide member 020 having a U-shaped section is supported on a base member 018 by two pivots 019 arranged in a front and rear direction (direction perpendicular to the surface of the drawing) so as to rock about the pivots 019 in a plane parallel with the surface of the drawing. When a steering wheel (not shown) is steered to the right, for example, movement of the steering wheel is transmitted to the guide member 020 through a Bawden wire 017r to move the guide member 020 to the right. When a guide slot 020a formed in an upper plate of the guide member 020 comes into contact with the mixing lever 016, the mixing lever 016 is moved to the right.
Thus, the mixing lever 016 receives an operation quantity relating to speed and running direction (advance or reverse) of the vehicle due to rotation of the change lever 010 and an operation quantity relating to turning direction due to steering of the steering wheel, synthesizes these two operation quantities and stops its movement at a position corresponding to the synthesized operation quantity.
In the meantime, push-pull rods 022r, 022l pivotally supported on the right and left side of the mixing lever 016 by inner ball joints 021, 021 rotates control arms 024r, 024l rotationally connected to the push-pull rods 022r, 022l by outer ball joints 023, 023, about pivotal shafts 026, 026 erected on a support plate 025. The rotations of the control arms 024r, 024l are stopped at positions corresponding to the stop position of the mixing lever 016.
The control arms 024r, 024l have front side ball joints 027, 027 at middle portions and ends of push-pull rods 028r, 028l are connected at positions of the ball joints 027, 027. Other ends of the push-pull rods 028r, 028l are connected to swash-plate rocking shafts of right and left static hydraulic pressure continuously variable transmissions (not shown) so that the right and left swash-plate rocking shafts are rotated by a quantity corresponding to a quantity of rotation of the control arms 024r, 024l until they stop. As the result, the vehicle undergoes a speed change operation such that wheels on the inner side of turning are rotated slower and wheels on the outer side of turning are rotated faster.
The above-mentioned speed change operation device has many rotation support shafts or rotation support shaft portions such as the rotary shaft 013, the pivot pin 015, the pivot 019, the inner side ball joints 021, 021, the outer side ball joints 023, 023, pivotal shafts 026, 026 and the front side ball joints 027, 027 which pivotally support respective many movable members such as the bracket 014, the mixing lever 016, the guide member 020, the push-pull rods 022r, 022l and the control arm 045r, 045l, and these rotation support shafts cross each other three-dimensionally and in a complicated manner so that rotary wear at respective parts and rocking or rotational shaking of the movable members is liable to occur. Therefore, the above speed changing operation device is inferior in respect of operation, wear resistance and control accuracy, and large-sized.